A series of studies will investigate the precedence effect, a phenomenon in which the sound arriving first at the ears is given more weight than later-arriving sounds in determining localization. The listener's perception of a sound's apparent location is determined not only by the actual location of the original sound, but also by echoes from reflecting surfaces. Onset and ongoing directional information may conflict in long duration sounds, producing ambiguity. In the proposed experiments sound will be presented to human adults in an anechoic chamber and over earphones in order to control timing and intensity of signals at both ears. Variations in the echo signal and onset/ongoing components will be introduced to discover how listeners assign location to sound sources when information is ambiguous. The listener's task is to detect the presence and location of the echo. Echo suppression is enhanced when the original sound and its echo are repeated; that is, echo threshold changes as a function of the ongoing stimulation. Variations in delay, spatial location, spectral content, and intensity of the repeated sound will be tested to determine if they affect this dynamic process. The ultimate goal of this project is to understand how listeners can accurately localize sound when highly conflicting cues reach their ears, a situation that occurs has relevance for both normal and hearing-impaired listeners in noisy backgrounds because localization is degraded in both cases by conflicting cues, which is then further exacerbated by hearing loss.